The braking systems used on a large commercial aircraft must be capable of dissipating tremendous kinetic energy as the aircraft is slowed during landing, yet provide smooth control of the plane's speed while it is taxiing on the runway. These requirements represent different considerations in braking system design and choice of material used in the brakes, and present the brake design engineer with a difficult choice-- to design a braking system optimized for slowing an aircraft traveling at high speed or a system offering smooth control.
For example, it is well-known that carbon brake materials are better able to withstand the heat build-up that occurs during landing when relatively high braking forces are applied to slow the aircraft. Furthermore, compared to steel brakes, carbon brakes provide a net reduction in airplane weight of as much as 1,600 pounds. However, carbon brakes are particularly susceptible to a significant variation in braking torque resulting from a given applied force. The torque may vary by a factor of up to 8 to 1, making it very difficult to smoothly control aircraft braking, especially at low taxiing speeds. The variation in the coefficient of friction of carbon brakes is a function of wheel speed, temperature, age, moisture and other factors. Of these factors, the variation in braking torque resulting from changes in wheel speed is perhaps the most significant. Like most other brake materials, carbon brakes tend to exhibit a much higher braking torque or coefficient of friction at lower wheel speeds, causing grabbing to occur when the brakes are applied at relatively low taxiing speeds. Due to complaints received from pilots concerning this problem, aircraft brake design engineers have been reluctant to specify carbon brakes.
In attempting to resolve this problem, brake pressure metering systems have been developed which incorporate a manual metering valve that has a lower pressure gain (the ratio of the force applied at the brakes relative to the force applied by the pilot on the brake pedal) at lower metering pressures than at higher metering pressures. The metering valve is intended to compensate for the higher brake torque developed at lower wheel speeds. However, the metering valve does not take into account the wide variations in braking torque that may occur, and which is most objectionable with respect to carbon brakes.
A device for controlling a braking system by sensing the actual torque exerted by the brake on a wheel is disclosed in U.S. Pat. No. 4,043,607. This device compares the torque developed by a brake against a predetermined value, and if the torque is greater than the value, actuates a servo control to reduce the torque accordingly. During the initial application of the brakes and during the time that the brake cylinders are filling with fluid, the servo control permits direct braking to occur. After the pressure of the fluid climbs to a predetermined level, the servo control is operative to limit the braking torque to a level determined as a function of the displacement of the brake pedal, according to one of two relationships or laws selected as a function of wheel speed. The brake control system described in this patent has several disadvantages. It does not include a mechanical metering valve and is not suitable for retrofit on an aircraft that uses a mechanical brake metering valve. Furthermore, in aircraft having an antiskid braking system, the antiskid control must inject an artificial over-torque signal into the torque limiting control circuit to effect an antiskid brake fluid pressure reduction. The combined antiskid and brake torque limiting control system is thus much more complicated, and it is impractical for this reason to retrofit an aircraft having an antiskid system with the prior art brake torque limiting servo control system described in this patent. Also, the characteristics of the individual servo valves used on the brakes of each wheel of a multiple brake landing gear must be closely matched to ensure that equal pressure is exerted at each wheel in response to the same control currents, to avoid imbalanced braking while operating in the open loop, direct control mode. Providing closely matched servo valves adds considerable cost to a brake control system of the type described in this patent.
Another torque limiting system is disclosed in U.S. Pat. No. 4,412,291. In this patent, a conventional mechanical metering system with antiskid valves is provided with a brake torque sensor and an electronic controller. The controller actuates the antiskid valves to reduce braking torque in response to the greater of a torque limiting signal and the antiskid control signal. The braking torque is limited only if it approaches a predetermined upper limit, which if exceeded, might damage the aircraft structure. The controller does not control braking torque during taxiing conditions and is not a solution to the unpredictable torque levels characteristic of carbon brakes.